The effect of beta-stimulants on mua channels in the frog ventrieular cells

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This experiment was designed to study the commitment of A-kinase system in the regulation of Na channel functions in the frog ventricular cells. The voltage clamp method using oilgap method was used to improve the spatical control of the membrane potential. 10 u M propranolol, beta-stimulant, does not change the transient membrane current through Na channel, I_<Na>, neither does so another stimulant, histamine. Forskoline, direct stimulant to adenylate cyclase, also fails to affect it. There are several reports that beta-stimulant suppressed I_<Na> via A-kinase system in the mammalian ventricular cells. One of the most serious problems in studying the change in INa for a long time is the shift of the voltage-dependency of inactivation process to the hyperpolarizing direction. This shift may spuriously produce a change in I_<Na> similar to those reported. Care was exercis in selecting the preparasions which do not produce the shift, comparing the inactivation curve before and after the … More stimulation of A-kinase system. Other concerning point is the expermental method. Because half of the cell membrane of a single cell is disrupted, critical malerials may be washied out from the intracellular phase, leading to an artificial loss of the control of channel. I have negated this possibility by confirming that beta-stimulant only enhances current through Ca channel, I_<Ca> in the experimental condition in which both I_<Na> and I_<Ca> are functioning. In spite of the conservative property in the amino acid sequence of the critical part of the Na channel, differdnce in the sensiivity of Na channel to mu-conotoxin between the skeletal and the nerve cells suggests a subtle change in molecular configuration. Thus, distinction of the response of Na channel to beta-stimulant may be due to differince in species. There remains possibility that this is due to an artifact, because the experimental condition in the mammalian preparatins is very strict and the extent of suppression of I_<Na> reported is very limited. All these experimental findings lead me to the conclusion that Na channels in the frog ventricular cells are not regulated by A-kinase systms. Less